Severe chronic cardiac insufficiency arising from cardiac disease or injury shortens and degrades the quality of life of many patients. One form of severe chronic cardiac insufficiency, congestive heart failure, is a pathophysiological state in which cardiac output is inadequate to meet physiological requirements of the body. The mortality rate for congestive heart failure is greater than 50% within 5 years of onset. Treatments for severe chronic cardiac insufficiency include heart transplants, artificial heart implants and cardiomyoplasty. Cardiac transplantation, using cyclosporine to inhibit tissue rejection, is a very successful technique for prolonging a cardiac patient's life, improving the survival rate to 80% at 1 year. However, the transplant operation is very expensive and heart availability is limited. The artificial heart has had very limited success.
Dynamic cardiomyoplasty is a surgical and electrical therapeutic technique in which a skeletal muscle flap is dissected from a patient, while maintaining its innervating neural tissues and neurovascular structures, and surgically placed around the patient's heart. A bioelectrical stimulation device with an electrical pulse generator and intramuscular electrodes are implanted which perform muscle electrical stimulation in synchrony with ventricular systole to support cardiac pumping. In functional electrical stimulation (FES) applications, one or more intramuscular electrodes are placed within a muscle body. Because intramuscular electrodes are firmly positioned with respect to nerve branches, they are advantageous for promoting long-term functional stability, giving rise to repeatable contractions.
A protocol of sequential and progressive skeletal muscle electrical stimulation causes glycolytic muscle fibers, predominant in skeletal muscle, to take the form of oxidative fibers. Oxidative fibers are resistant to fatigue and have histochemical and biochemical characteristics of myocardium. Stimulated skeletal muscle transforms into a fatigue-resistant state suitable for chronic ventricular assistance, enabling dynamic cardiomyoplasty. The skeletal muscle is then trained to function in the manner of cardiac muscle to assist the heart in increasing the patient's cardiac output. This permits skeletal muscle assistance of a patient's ailing heart muscle.
G. J. Magovern, in U.S. Pat. No. 4,791,911, entitled "Method of Cardiac Reconstructive Surgery", issued Dec. 20, 1988, discloses a surgical method of reconstructing damaged cardiac muscle using a latissimus dorsi skeletal muscle autograft.
J. C. Chachques et al., in U.S. Pat. No. 4,735,205, entitled "Method and Apparatus including a Sliding Insulation Lead for Cardiac Assistance", issued Apr. 5, 1988, discloses a suitable electrode configuration used in a system which provides muscle stimulation for a muscle surgically adapted to perform myocardial substitution. The leads have variable electrode surface areas which are adjusted at the time of surgical implantation to expose a desired length of electrode surface area extending through the muscle. At implantation, a tubular insulation sheath which overlies a conductor within the lead may be extended over or retracted from the conductor to expose a desired length of conductor. The exposed conductor acts as a muscle stimulation electrode.
P. A. Grandjean et al., in U.S. Pat. No. 5,009,229, entitled "Steroid Eluting Intramuscular Lead", issued Apr. 23, 1991, describes an intramuscular lead which is adapted from a type of cardiac pacing lead. This lead includes an insulated wire body, an electrode, a strand of suture material extending the entire length of the lead and distal to the electrode, and a curved needle attached to the distal end of the strand of suture material. The suture material is treated with a steroid drug, such as glucocorticosteroid, which, upon chronic implantation, is eluted from the suture material to treat tissue inflammation or damage caused by the implantation procedure or subsequent irritation. Although this lead may avoid or lessen damage to nerves caused by electrical stimulation, it does not diminish the high current flows which are required to stimulate muscle contraction.
In the aforementioned devices and methods, skeletal muscles have been employed to augment cardiac performance in patients with dilated or ischemic cardiomyopathy. In the procedure of cardiomyoplasty, the patient's latissimus dorsi muscle is wrapped around the heart and stimulated to contract during systole, in synchronization with the heart. In aortomyoplasty, the latissimus dorsi muscle is wrapped around the aorta and stimulated to contract during cardiac diastole. The latter technique is termed counterpulsation. Other manifestations of counterpulsation include a skeletal muscle ventricle technique in which the latissimus dorsi muscle is wrapped around a small diameter cylinder or cone. The muscle pump is then connected into the circulatory system and stimulated to contract during diastole to augment the diastolic blood pressure.
In all of these known procedures, only one channel of neuromuscular stimulation is employed. Therefore, all of the stimulated muscle is activated at one time to contract and compress the blood reservoir within the heart, the aorta or the skeletal muscle ventricle employed in the procedure. Because the entire muscle contracts at the same time, the force of contraction does not propel the blood in a particular direction. Instead, as a result of the contraction, the blood will flow away from the arteries as well as toward the arteries.
The success or failure of a cardiac assistance procedure is judged by measurements of such parameters as cardiac output, ejection fraction, stroke volume, peak pressure, contractility indices and end-diastolic volumes. Results of such examinations to date have shown only minor quantitative improvement resulting from the cardiomyoplasty procedure. (See "The Allegheny Hospital Experience", Magovern G. J. et al., in Cardiomyoplasty, pp. 159-170, eds. Carpentier A. et al., Futura, 1991.) One reason for the failure to detect a strong quantitative improvement during a skeletal muscle cardiac assistance procedure may be the failure to direct the flow in a desired direction, toward the arteries.